<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>行内元素/行内块元素水平居中对齐</title>
    <style>
      .header {
          width: 900px;
          height: 200px;
          background-color: pink;
          text-align: center;
          margin: 100px auto;
      }
      /* 行内元素或者行内块元素水平居中给其父元素添加 text-align:center 即可 */
    </style>
</head>
<body>
    <div class="header">
        <span>里面的文字</span>
    </div>
    <div class="header">
        <img src="./images/img.jpg" alt="">
    </div>
</body>
</html>